Power rectifier circuit

ABSTRACT

A mains rectifier circuit for power supply units, in particular for switched-mode power supplies, having a full-wave rectifier (D1 . . . D4) which is connected to the mains voltage and having a smoothing capacitor (CL) which is connected downstream of said full-wave rectifier (D1 . . . D4), the smoothing capacitor being charged at a clock frequency, which is considerably higher than the mains frequency, via the series circuit formed by a boost inductance (LB) and a diode (DS), in the case of which series circuit the junction point between the boost inductance and the diode is connected to earth via a controlled switch (SB), the smoothing capacitor (CL) and the load (SNT) are connected to the full-wave rectifier (D1 . . . D4), an auxiliary rectifier (D1&#39;, D2&#39;) is provided, from which the series circuit formed by the boost inductance and the diode leads to the smoothing capacitor, and the switch is controlled via a drive circuit (AB) such that additional charging current is supplied to the smoothing capacitor, via the boost inductance, in time periods around the zero crossings of the rectified half-cycles.

FIELD OF THE INVENTION

The present invention relates to a mains rectifier circuit for powersupply units, in particular for switched-mode power supplies.

BACKGROUND INFORMATION

Power supply units, in particular switched-mode power supplies forconnection to an AC mains, in the simplest case have a rectifierdownstream of which an electrolytic capacitor is connected. The chargingcurrent profile of the capacitor leads to severe harmonics on the mainsand loads said mains with a high apparent power. The mass use of powersupply units within the last twenty years has for these reasons alreadyled to serious problems in the power supply grids.

A brief presentation of these problems and various solution proposals,which are intended to lead to a balanced mains load, can be found in thearticle "Comparison of Standards and Power Supply Design Options forLimiting Harmonic Distortion in Power Systems", by Key and Lai, in "IEEETransactions on Industry Applications" Vol. 20, No. 4, July/August 1993,pages 688 to 695. One of the circuits which are illustrated there andare also used in practice has a so-called "boost converter". A virtuallysinusoidal input current profile can even be achieved using said boostconverter, by means of a complex controller. As is noted in the article,however, the relatively high costs and relatively poor reliability,which is governed by additional components, preclude widespreadintroduction to the market.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a simple andcost-effective solution which leads to a marked reduction in theharmonic content and to an improvement in the power factor.

This object is achieved by a circuit of the type having a full-waverectifier which is connected to the mains voltage and having a smoothingcapacitor which is connected downstream of said full-wave rectifier, thesmoothing capacitor being charged at a clock frequency which isconsiderably higher than the mains frequency, via a series circuitformed by a boost inductance and a diode. The junction point between theboost inductance and the diode is connected to earth via a controlledswitch in which, according to the invention, the smoothing capacitor andthe load are connected to the full-wave rectifier, an auxiliaryrectifier is provided, from which the series circuit formed by the boostinductance and the diode leads to the smoothing capacitor, and theswitch is controlled via a drive circuit such that additional chargingcurrent is supplied to the smoothing capacitor, via the boostinductance, in time periods around the zero crossings of the rectifiedhalf-cycles.

While current peaks occur only in the region of the peak values of therectified half-cycles in the case of a pure rectifier circuit with asmoothing capacitor--in the normal load case--part of the chargingcurrent is shifted, as a result of the invention, using simple means,specifically into those regions around the zero crossings in which nocurrent otherwise flows. In this case, the cost for additionalcomponents is absolutely kept within limits. A reduction in the harmoniccontent of the input current is achieved, as well as an improvement inthe efficiency and the power factor.

It is advantageous if an auxiliary capacitor is connected directlydownstream of the auxiliary rectifier. The clock frequency of thecontrolled switch is expediently in the kHz range.

The auxiliary rectifier is advantageously formed by two diodes which areconnected as a half-bridge with respect to the full-wave rectifier.

The object is also achieved by a mains rectifier circuit for powersupply units, in particular for switched-mode power supplies, having afull-wave rectifier which is connected to the mains voltage and having asmoothing capacitor, the smoothing capacitor being charged at a clockfrequency, which is considerably higher than the mains frequency, viathe series circuit formed by a boost inductance and a diode, in the caseof which series circuit the junction point between the boost inductanceand the diode is connected to earth via a first controlled switch, thecharging voltage being selected to be higher than the peak value of themains voltage, in the case of which, according to the invention, theload is connected to the full-wave rectifier, an auxiliary rectifier isprovided, from which the series circuit formed by the boost inductanceand the diode leads to the smoothing capacitor, and the first switch iscontrolled via a drive circuit such that charging current is supplied tothe smoothing capacitor, via the boost inductance, in time periodsaround the zero crossings of the rectified half-cycles, and such that asecond controlled switch is provided via which the load is connected tothe smoothing capacitor and which is controlled via a second drivecircuit such that it closes periodically at the rate of twice the mainsfrequency in a time period around the zero crossings of the rectifiedhalf-cycles. The use of the second switch, which is pulsed insynchronism with the mains, results in a further improvement in theinput current profile in the sense of a reduced harmonic content.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic circuit diagram of a first embodiment of thepresent invention.

FIGS. 2a and 2b show the time profiles of the output voltage and of theinput current, respectively, for the configuration according to FIG. 1.

FIG. 3 shows a schematic circuit diagram of a second embodiment of thepresent invention.

FIGS. 4a, 4b and 4c show the time profiles of the output voltage and ofthe output current, respectively, in two load cases for theconfiguration according to FIG. 3.

DETAILED DESCRIPTION

FIG. 1 shows a mains rectifier circuit for a switched-mode power supplySNT which is only indicated here and is also called the "load" in thefollowing text. A full-wave rectifier D1 . . . D4 is connected to themains voltage UE and is composed of four diodes D1, D2, D3, D4 which arearranged in a bridge circuit. The load, or the switched-mode powersupply SNT, as well as a smoothing capacitor CL are connected directly,possibly via a filter or filter element as well, to the output of thefull-wave rectifier D1 . . . D4. An auxiliary rectifier, comprising twoadditional diodes D1', D2' and the diodes D3, D4, is also provided. Theoutput of this auxiliary rectifier is connected to the smoothingcapacitor CL via the series circuit formed by a boost inductance LB anda diode DS which is arranged in the forward direction. An auxiliarycapacitor CH is also connected directly downstream of the auxiliaryrectifier D1', D2'.

The junction point between the boost inductance LB and the diode DS isconnected to earth via a controlled switch SB. This switch SB iscontrolled via a drive circuit AB such that the smoothing capacitor CLis additionally charged, in time periods around the zero crossings ofthe rectified half-cycles, at a clock frequency which is considerablyhigher than the mains frequency, the charging voltage being selected tobe higher than the peak value of the mains voltage.

The switch SB can be designed, for example, as a field-effect transistorand it is pulsed at a frequency in the kHz range, typically between 20kHz and 100 kHz, but not, as normal, over the entire half-period butonly for a preselected time around the zero crossings. In consequence,the rated power of the series inductor, in this case the boostinductance LB, as well as that of the transistor switch SB and also thepower loss of the entire circuit are reduced. The harmonic content ofthe input current IE can be greatly reduced in comparison with thesimple bridge rectifier circuit with smoothing capacitor since a currentflow is now also achieved other than at the voltage maxima and thecurrent peaks in the region of the voltage maxima can be correspondinglyreduced.

FIG. 2a shows the profile of the output voltage UA for a specific loadcase, the rectified half-cycles being shown in dashed-dotted form. Theassociated input current can be seen in FIG. 2b. The current pulses inthe region of the voltage maxima (charging current via the rectifier D1. . . D4) can be seen there, as well as the triangular current pulsesaround the voltage minima, which are caused by the effect of the boostcircuit.

If one is prepared to accept somewhat more cost in order to achieve afurther improvement in the mains load, the circuit according to FIG. 3can be used, which additionally has a second switch which is pulsed insynchronism with the mains.

In the same way as in FIG. 1, the load or the switched-mode power supplySNT is connected directly, possibly via a filter or filter element aswell, to the output of the full-wave rectifier D1 . . . D4. The outputof the auxiliary rectifier D1', D2' (D3, D4) is connected to thesmoothing capacitor CL via the series circuit formed by the boostinductance LB and the diode DES. Once again, an auxiliary capacitor CHis connected directly downstream of the auxiliary rectifier D1', D2'here. The junction point between the boost inductance LB and the diodeDS is connected to earth via a first controlled switch SB. This firstswitch SB is controlled via a first drive circuit AB such that thesmoothing capacitor CL is charged, in time periods around the zerocrossings of the rectified half-cycles, at a clock frequency which isconsiderably higher than the mains frequency, the charging voltage beingselected to be higher than the peak value of the mains voltage.

The smoothing capacitor CL is connected to the load SNT via a secondcontrolled switch ST which is controlled via a second drive circuit ATsuch that it closes periodically at the rate of twice the mainsfrequency in a time period around the zero crossings of the rectifiedhalf-cycles. The method of operation of the boost circuit LB-SB-DS hasalready been explained in conjunction with FIG. 1.

The smoothing capacitor CL is discharged to the load SNT by means of thesecond controlled switch ST during a selected sub-period of thehalf-period around the zero crossings of the rectified half-cycles, suchthat the switch ST closes and opens periodically at the rate of twicethe mains frequency, for example +/-45° around the voltage zerocrossing, that is to say part of the current flow now additionallyoccurs before and after the peaks of the normal rectifier circuit.

The switch ST, which is pulsed at twice the mains frequency, acts in thesense of broadening the current pulses in the region of the voltagemaxima. Overall, a current profile is achieved which is distributed wellover the period duration, as can be seen from FIG. 4. FIG. 4a shows theprofile of the output voltage, and one can clearly see the voltageovershoot at CL in the region of the zero crossings, caused by thestep-up converter LB, SB, DS, CL (boost converter). FIGS. 4a and 4brelate to the operating case with "100% rated load" and it can be seenfrom FIG. 4b that input current always flows--except at the zerocrossings. FIG. 4c relates to a load of only 25% of the rated load. Inthis case, there are time intervals during which no input current flows,but the current overall is lower here.

The circuit according to FIG. 3 thus produces a further improvement withrespect to a low harmonic content and the power factor, without anyconsiderable additional cost. The controlled switches SB, ST each haveto carry only part of the total switching power since part of thecurrent flow to the load also takes place directly via the rectifierbridge. More cost-effective types can therefore be selected for thepower semiconductors, overall.

What is claimed is:
 1. A mains rectifier circuit for power supply units,comprising:a full-wave rectifier connected to a mains voltage having amain s frequency and directly connected to a load; a smoothing capacitorconnected downstream of the full-wave rectifier, the smoothing capacitorbeing charged at a clock frequency, the clock frequency being higherthan the mains frequency; a series circuit formed by a boost inductanceand a diode, a junction point between the boost inductance and the diodebeing connected to earth via a controlled switch, the smoothingcapacitor being charged via the series circuit, the controlled switchbeing controlled via a drive circuit such that additional chargingcurrent is supplied to the smoothing capacitor, via the boostinductance, in time periods around zero crossings of the rectifiedhalf-cycles; and an auxiliary rectifier coupled to the smoothingcapacitor via the series circuit.
 2. The mains rectifier circuit ofclaim 1, wherein an auxiliary capacitor is connected directly downstreamof the auxiliary rectifier.
 3. The mains rectifier circuit of claim 1,wherein the clock frequency is substantially 1 kHz or greater.
 4. Themains rectifier circuit of claim 1, wherein the auxiliary rectifiercomprises two diodes which are connected as a half-bridge with respectto the full-wave rectifier.
 5. A mains rectifier circuit for powersupply units, comprising:a full-wave rectifier connected to a mainsvoltage having a mains frequency and directly connected to a load; asmoothing capacitor connected downstream of the full-wave rectifier, thesmoothing capacitor being charged at a clock frequency, the clockfrequency being higher than the mains frequency; a series circuit formedby a boost inductance and a diode, a junction point between the boostinductance and the diode being connected to earth via a first controlledswitch, the smoothing capacitor being charged by a charging voltage viathe series circuit, the charging voltage being higher than a peak valueof the mains voltage, the first controlled switch being controlled via adrive circuit such that additional charging current is supplied to thesmoothing capacitor, via the boost inductance, in time periods aroundzero crossings of the rectified half-cycles; a second controlled switchconnecting the load to the smoothing capacitor and controlled via asecond drive circuit such that the second controlled switch closesperiodically at a rate of twice the mains frequency in a time periodaround zero crossings of the rectified half-cycles; and an auxiliaryrectifier coupled to the smoothing capacitor via the series circuit. 6.The mains rectifier circuit according to claim 1, wherein the auxiliaryrectifier and the series circuit formed by the boost inductance and thediode are arranged in parallel to the full-wave rectifier.